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Well if an electronic ignition is in place.A thermostat triggers the control box to start the exhaust fan,the fan triggers a pressure switch that tells the control box to start a spark and to open the pilot gas part of the gas valve.Once pilot is established a flame sensor picks up an electronic signal the goes to the control box which triggers the main gas valve to open.
I would say about 7-8 times out of 10 that if the pilot comes on and the main gas valve does not come on it is the flame sensor which is at fault.Since the electronic signal that it senses is such a small voltage,any small coating on the sensor will slow or prevent that very small electronic signal from entering that flame sensor and thus prevent the main gas valve from opening.